The present invention relates to user controls for an oscilloscope, and in particular to control of horizontal display modes in an oscilloscope.
Earlier analog oscilloscopes included a trigger circuit which analyzed an acquired input signal for a user selected event, e.g. a negative-going or positive-going transition at a user selected voltage level, and in a specified channel if the oscilloscope was a multichannel oscilloscope. When the trigger event occurred, a waveform representing the acquired input signal occurring from that point forward in time was displayed on the display screen of the oscilloscope. The trigger point was displayed as the leftmost point in the displayed waveform. The timing of the displayed waveform was controlled by the user, and could be changed. As the timing changed, the time (horizontal) magnification of the waveform also changed. The change was made relative to the trigger point, which remained fixed at the leftmost point of the displayed waveform.
FIG. 1 is a combined waveform and oscilloscope display diagram illustrating the trigger and horizontal magnification characteristics of an analog oscilloscope. In FIG. 1, a waveform 10 represents e.g. the voltage of an acquired input signal supplied to the oscilloscope via an oscilloscope probe. A user has set the trigger to the illustrated trigger point 12 in a known manner. A first display 11 is a waveform display on the display screen of the oscilloscope in which the time base is set to display the waveform 10 from the trigger point 12 to the point 14. A second display 13 is a waveform display on the display screen of the oscilloscope in which the trigger point is the same as that in the first display 11, but the time base has been adjusted to display the waveform 10 from the trigger point 12 to the point 16.
It is apparent from FIG. 1 that the trigger point 12 is displayed in the waveform display, regardless of the setting of the time base, and that the trigger point 12 is always displayed as the leftmost point of the waveform display. Furthermore, it is apparent that the displayed waveform is magnified from the trigger point 12 on. That is, the trigger point is co-located with the magnification point. Consequently, the waveform display may be magnified horizontally to any desired degree without losing the display of the trigger point 12.
As digital oscilloscopes were developed, it became possible to display portions of the waveform occurring before the trigger point, as well as after. The trigger could be set by a user in the same manner as in analog oscilloscopes. However, instead of displaying the trigger point as the leftmost point in the displayed waveform. The trigger point could be displayed at any point in the displayed waveform. Also as with analog oscilloscopes, the timing of the displayed waveform was controlled by the user, and could be changed, thus changing the time (horizontal) magnification of the waveform. Again, as with analog oscilloscopes, the change was made relative to the trigger point, but in digital oscilloscopes, the trigger point was not constrained to be the leftmost point in the displayed waveform.
FIG. 2 is a combined waveform and oscilloscope display diagram illustrating the trigger and horizontal magnification characteristics of a digital oscilloscope. In FIG. 2, again a waveform 10 represents e.g. the voltage of an input signal supplied to an oscilloscope via an oscilloscope probe. A user has set the trigger to the illustrated trigger point 12xe2x80x2 in a known manner. This trigger point 12xe2x80x2 is different than that set in FIG. 1, however. A first display 21 is a waveform display on the display screen of the oscilloscope in which the time base is set to display the waveform 10 from the point 22 to the point 24, and includes the trigger point 12xe2x80x2 within the displayed waveform. A second display 23 is a waveform display on the display screen of the oscilloscope in which the trigger point 12xe2x80x2 is displayed in the same position as that in the first display 21, but the time base has been adjusted to display the waveform 10 from the point 26 to the point 28.
It is apparent from FIG. 2 that, as in FIG. 1, the trigger point 12xe2x80x2 is displayed in the waveform display, regardless of the setting of the time base. However, in FIG. 2, the trigger point 12xe2x80x2 is displayed within the waveform display, not at the leftmost edge, as in FIG. 1. It is also apparent that, as in FIG. 1, the magnified signal in display 23 is magnified about the location of the trigger point 12xe2x80x2. Thus, the trigger point is co-located with the magnification point. That is, the trigger point remains in the same place in the waveform display, while the displayed waveform expands or contracts around that point as the time base is changed by the user. Consequently, the waveform display may still be magnified horizontally to any desired degree without losing the display of the trigger point.
Other digital oscilloscopes used a different technique for triggering the waveform display, termed a viewport technique. In such a digital oscilloscope, the displayed waveform is displaced by some user controlled amount of time from the trigger point. This display is termed a viewport. The displayed waveform in the viewport does not, necessarily, contain the trigger point. The magnification of the displayed waveform takes place within the viewport and the displayed waveform expands and contracts around a magnification point, which may be any point on the display screen, such as the leftmost point of the viewport. In a preferred embodiment, however, the magnification point is the center point of the display screen.
FIG. 3 is a combined waveform and oscilloscope display diagram illustrating the trigger and horizontal magnification characteristics of a digital oscilloscope. In FIG. 3, again a waveform 10 represents e.g. the voltage of an input signal supplied to an oscilloscope via an oscilloscope probe. A user has set the trigger to the illustrated trigger point 12 in a known manner. This trigger point 12 is the same that set in FIG. 1. In FIG. 3, a user has set a time displacement to point 32 of the input signal. This is a point after the trigger point, and is referred to as a positive time displacement. It is also possible for a user to specify a negative time displacement to a point before the trigger point. A first display 31 is a waveform display on the display screen of the oscilloscope in which the time base is set to display the waveform 10 starting from the time displacement point 32 to the point 34. The center point of the display screen corresponds to point 35 of the waveform 10. A second display 33 is a waveform display on the display screen of the oscilloscope in which the time base has been adjusted to display the waveform 10 from the point 36 to the point 38. The center point of the display screen continues to correspond to point 35 of the waveform 10. The point 35, thus, is the magnification point.
It is apparent from FIG. 3 that the trigger point 12 is not necessarily displayed in the waveform display, depending upon the settings of the time displacement and the time base. Also, in FIG. 3, the magnified signal in display 33 is magnified about the center point of the waveform display. Thus, in FIG. 3, the trigger point 12 is not co-located with the magnification point 35. Instead, the displayed waveform expands or contracts around the magnification point 35, which corresponds to the display screen center point, as the time base is changed by the user.
One skilled in the art will understand that more sophisticated triggering techniques exist. For example, there may be what is termed a main trigger, which analyzes an acquired input signal for a user specified main trigger event, and a secondary, or delayed trigger. These operate in the following manner: after the main trigger detects the main trigger event, the secondary trigger analyzes the acquired input signal for a user specified secondary or delayed trigger event. This secondary trigger event then initiates the waveform display. The oscilloscope may be controlled to display the waveform after, or surrounding, the main trigger event; or after, or surrounding, the secondary or delayed trigger event.
Alternatively, the oscilloscope may include what is termed an A and a B trigger. The A trigger may be set by the user in the known manner, e.g. a positive going or negative going signal through a user set voltage level on a selected channel. The B trigger may be similarly set. The combination of the A trigger and the B trigger initiates the waveform display. Either or both of the A and B triggers may also have a time displacement associated with them. Additional such triggers are also possible, e.g. C trigger, D trigger, etc. In any case, using the waveform display techniques illustrated in FIG. 1 or 2, the displayed waveform contains the trigger event, and the magnification occurs around the display point representing the time of that event. Using the waveform display technique illustrated in FIG. 3, the trigger point is not necessarily contained in the displayed waveform, and the displayed waveform is time magnified about the magnification point, which is the center point of the display screen in a preferred embodiment.
The trigger-centric techniques of FIGS. 1 and 2 are useful for a user to observe phenomena surrounding the trigger point because, regardless of the time magnification, the trigger point is never removed from the waveform display. For example, in manufacturing or production environments, in which signals containing trigger events can be reliably and accurately supplied to and detected by the oscilloscope, the trigger-centric technique allows observation of a signal near such trigger points. On the other hand, the viewport technique of FIG. 3 is useful to observe phenomena located at some time distance from the trigger point. For example, a location in the middle of a pulse train can be easily observed by adjusting the trigger to detect the start of the pulse train, and adjusting the time displacement to the location of interest in the middle of the pulse train.
In existing oscilloscopes, to switch from a trigger-centric waveform display to a viewport waveform display requires resetting of trigger controls on the oscilloscopes, which can involve properly setting several switches and making appropriate adjustments to user controls for trigger voltage levels and time displacements for each trigger. It is desirable to provide a simple and fast way for a user to switch between the trigger-centric and the viewport modes of operation so that the user can observe the acquired input waveform both in the location of the trigger to ensure it is operating reliably and accurately, and easily switch to observe the acquired input waveform in the time location of the delayed phenomenon.
In accordance with principles of the present invention, a system for displaying waveforms representing an input signal includes a display subsystem, coupled to a source of the input signal, for displaying a waveform representing the input signal in response to a display control signal. A trigger circuit is coupled to the input signal source and detects a trigger event. A time displacement circuit is coupled to the trigger circuit and generates a time displaced trigger signal a controllable amount of time after detection of the trigger event. A switch is coupled between the trigger circuit, the time displacement circuit, and the display subsystem. The switch selectively generates the display control signal in response to either the detected trigger event or the time displaced trigger event.